The following three general methods are known for producing magnetite, and it is known that they have respective drawbacks as described below.
(1) Wet process: an aqueous solution of Fe.sup.2+ +2 Fe.sup.3+ is rendered alkaline and coprecipitated. PA0 Drawbacks: production cost is high and it is not suitable for mass production, although fine particles each having a relatively high purity are readily obtained by coprecipitation. PA0 (2) Dry process: hematite is heated in either hydrogen and carbon monoxide or steam to thereby be reduced. PA0 Drawbacks: the reaction is performed in a relatively highly reducing atmosphere, e.g., either high pressure steam or hydrogen and carbon monoxide, so that danger accompanies the mass production, and large scale equipment is required. PA0 (3) Process in which natural magnetite is pulverized. PA0 Drawbacks: a natural mineral is used as a starting material, so that it is difficult to stably produce high quality powder for a prolonged period of time.
Taking into account the advantages and disadvantages of the above processes, various proposals have been made with respect to the production of magnetite powder for use in magnetic toners and carriers for electrophotography, etc.
For example, Japanese Patent Publication Nos. 238,580/1987, 39,498/1990 and 51,505/1990 disclose processes for producing magnetite useful as a carrier component of an electrophotographic developer, especially a two-component developer composed of a toner and a carrier. Each of these processes comprises providing magnetite powder (or globular magnetite particles) as a starting material, granulating the same into globular grains through, for example, mixing with a binder, followed by heating (sintering), and applying a resin coating to the resultant globular magnetite particles according to the conventional technique.
The use of a material called soft ferrite as carrier particles is known (U.S. Pat. No. 3,929,657, etc.). In this connection, it is known that the carrier particles each composed of the above ferrite not only are excellent in magnetic properties but also do not require a resin coating layer, so that their durability is excellent. Noting that ferrite particles have resistance variations depending on the control of sintering atmosphere, even with the same composition, attempts have been made to enlarge resistance variation ranges by changing the sintering atmosphere. As an example thereof, there can be mentioned an attempt described in Japanese Patent Publication No. 37782/1987. This comprises providing an Mg ferrite as a starting material and increasing the amount of ferric oxide (Fe.sub.2 O.sub.3) to more than 53% by mole so as to enlarge its resistance variation range.
On the basis of the above prior art, the inventors have found a novel process for producing single phase magnetite as an oxidic magnetic material.